The invention relates to a display screen, in particular a color display screen for a cathode ray tube, having a phosphor layer comprising a phosphor composition of a phosphor coated with an oxygen compound of one of the elements magnesium, calcium, barium, zinc and aluminium, and with colloidal SiO.sub.2.
Display screens are provided on the inner surfaces thereof with a structured phosphor layer which contains one or more phosphors arranged in a dot pattern or stripe pattern. In the case of color display screens, the pattern consists of triplets of vertical stripes or triangularly arranged dots, each triplet containing phosphors in the primary emission colors red, blue and green.
This structured phosphor layer is customarily provided in a photochemical process which is based, in a wet-chemical process, on photo-sensitive phosphor suspensions or, in a dry process, on photo-sensitive phosphor lacquers.
The photo-sensitive phosphor suspensions for the wet-chemical process are composed of a suspension of the phosphor as a powder in water and of polyvinyl alcohol and ammonium dichromate as the photo-active system. These photo-sensitive suspensions are cured by exposure to UV light.
Said phosphor suspensions must be resistant to agglomeration and exhibit favorable sedimentation properties. However, the customarily used raw materials for the phosphors are difficult to suspend in aqueous systems, either because they are hydrophobic in character or because the phosphor grains have formed agglomerates. Therefore, to make them readily suspendible, the raw phosphor powders are subjected to a pretreatment in which the surface is modified; for example, they may be provided with a hydrophilic coating.
In U.S. Pat. No. 5,366,834, a description is given of a coating for improving the dispersibility of phosphors, which coating is applied to a first coating and comprises Zn, Al and/or an alkaline earth metal as well as colloidal silicon oxide, an aluminium oxide sol and/or a titanium-oxide sol having a particle size of 50 nm or less. In accordance with U.S. Pat. No. 5,366,834, column 7, lines 34 to 40, said coating should have a structure in which a hydroxide of Zn, Al and/or an alkaline earth metal with SiO.sub.2.nH.sub.2 O (n.gtoreq.0), Al.sub.2 O.sub.3.nH.sub.2 O (n.gtoreq.0) or TiO.sub.2.nH.sub.2 O are mixed and bonded to the first coating. It is also assumed that the coating is a silicate or an oxide which is formed when the metal ions and, for example, the colloidal SiO.sub.2 are concurrently precipitated.
The quality of display screens is to meet ever higher requirements in terms of brightness, color purity, absence of haziness and definition of the phosphor coating.